<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Document</title>
  <style>
    img {
      width: 400px;
      height: 300px;
    }
  </style>
</head>

<body>
  <div>
    <p></p>
    <img src="./img/img1.jpg" alt="" />
  </div>
  <script>
    const imgs = ["./img/img2.jpg", "./img/img3.jpg", "./img/img4.jfif", "./img/img5.jpg"];
    const img = document.querySelector("img");
    const test = document.querySelector("p");
    //  实现点击图片，切换下一张图片
    let index = 0;

    test.innerHTML = "我是第" + (index + 1) + "张图片";
    img.addEventListener("click", function () {
      if (index < imgs.length) {
        img.src = imgs[index];
        index++;
        test.innerHTML = "我是第" + (index + 1) + "张图片";
      } else {
        alert('没有了')
      }

      //  切换图片后，同时让浏览器提前加载下载一张图片
      if (index < imgs.length) {
        preLoad(imgs[index]);
      }
    });

    // 调用加载函数，页面一开始就加载数组第一个元素
    preLoad(imgs[0]);
    // 封装函数，新建一个img标签，然后增加src属性，让浏览器加载下一张图片
    function preLoad (src) {
      img.addEventListener("load", () => {
        // 创建一个新的img标签
        const img = document.createElement("img");
        // 给img添加src属性，为我们传进来的src
        img.src = src;
      });
    }
  </script>
</body>

</html>